1. Field of the Invention
This invention relates to novel coherently grown composites of two zeotypes. The coherently grown composites comprise two distinct crystal phases that are coherently and rationally joined together in a single material.
2. Description of the Related Art
Classes of molecular sieves include crystalline aluminophosphate, silicoaluminophosphate, or metalloaluminophosphate compositions which are microporous and which are formed from corner sharing AlO4/2 and PO4/2 tetrahedra. In 1982, Wilson et al. first reported aluminophosphate molecular sieves, the so-called AlPOs, which are microporous materials that have many of the same properties as zeolites, although they are silica free (See U.S. Pat. No. 4,310,440). Subsequently, charge was introduced to the neutral aluminophosphate frameworks via the substitution of SiO4/2 tetrahedra for PO4/2+ tetrahedra to produce the silicoaluminophosphate (SAPO) molecular sieves as described by Lok, et. al. (See U.S. Pat. No. 4,440,871). Another way to introduce framework charge to neutral aluminophosphates is to substitute [Me2+O4/2]2− tetrahedra for AlO4/2− tetrahedra, which yield the MeAPO molecular sieves (see U.S. Pat. No. 4,567,029). It is furthermore possible to introduce framework charge on AlPO-based molecular sieves via the simultaneous introduction of SiO4/2 and [M2+O4/2]2− tetrahedra to the framework, giving MeAPSO molecular sieves (See U.S. Pat. No. 4,973,785).
Numerous molecular sieves, both naturally occurring and synthetically prepared, are used in various industrial processes. Synthetically, these molecular sieves are prepared via hydrothermal synthesis employing suitable sources of Si, Al, P, and structure directing agents such as amines or organoammonium cations. The structure directing agents reside in the pores of the molecular sieve and are largely responsible for the particular structure that is ultimately formed. These species may balance the framework charge associated with silicon or other metals such as Zn in the aluminophosphate compositions and can also serve as space fillers to stabilize the tetrahedral network framework. Molecular sieves are characterized by having pore openings of uniform dimensions, having a significant ion exchange capacity, and being capable of reversibly desorbing an adsorbed phase which is dispersed throughout the internal voids of the crystal without significantly displacing any atoms which make up the permanent molecular sieve crystal structure. Molecular sieves can be used as catalysts for hydrocarbon conversion reactions, which can take place on outside surfaces as well as on internal surfaces within the pore.
Synthesis of molecular sieve materials often relies on the use of organoamino or organoammonium templates known as organic structure directing agents (OSDAs). While simple OSDAs such as tetramethylammonium, tetraethylammonium and tetrapropylammonium are commercially available, often, OSDAs are complicated molecules that are difficult and expensive to synthesize; however, their importance lies in their ability to impart aspects of their structural features to the molecular sieve to yield a desirable pore structure. For example, the use of 1,4,7,10,13,16-hexamethyl-1,4,7,10,13,16-hexaazacyclooctadecane as OSDA has been shown to allow synthesis of STA-7, an aluminophosphate based material of the SAV zeotype (Wright, et. al. J. Chem. Soc., Dalton Trans., 2000, 1243-1248); the use of 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane (‘Kryptofix 222’) led to the synthesis of AlPO4-42 (Schreyeck, et. al. Micro. Meso. Mater. 1998, 22, 87-106); MAPO-35, a magnesium aluminophosphate material with the LEV topology, is disclosed in U.S. Pat. No. 4,567,029 in which quinuclidine is employed as a structure directing agent; and in U.S. Pat. No. 4,973,785, the MeAPSO composition CoAPSO-35 is disclosed, which contains both cobalt and silicon in the framework in addition to Al and P and uses methylquinuclidine as the structure directing agent.
The art clearly shows that use of complex organoammonium SDAs often results in new molecular sieve materials. However, the synthesis of these complicated organoammonium compounds is quite lengthy and requires many steps, often in an organic solvent, thereby hindering development of the new molecular sieve material. Frequently, even for simple, commercially available OSDAs, the OSDA is the most costly ingredient used in synthesizing molecular sieve materials. Consequently, it would be economically advantageous to synthesize new molecular sieves from either commercially available organoammonium SDAs or SDAs which may be readily synthesized from commercially available starting materials.
The simple, commercially available, amine morpholine (tetrahydro-1,4-oxazine) has been previously utilized in aluminophosphate based molecular sieve synthesis and has been shown to yield CHA-type molecular sieves (Marchese, et. al. Micro. Meso. Mater. 1999, 30, 145-53; Ito, et. al. Acta Cryst. 1985, C41, 1698-1700), but has not yet been shown to yield other structure type molecular sieves. Additionally, the vapor pressure of morpholine is relatively high, making its use on commercial scale troublesome as low vapor pressure organoammonium SDAs are preferred.
The complicated OSDA(s) discussed previously were synthesized ex-situ and added to the reaction mixture at several points. However, one drawback of ex-situ synthesis is the process is typically carried out in the presence of an organic solvent, which necessitates at least one undesirable purification step to recover the SDA from the unwanted organic material.
The properties of molecular sieves are highly dependent on their crystal structure, as this can dictate how fast molecules can move through the pores, what molecules can be excluded from the pores, the number and strength of the acid sites, etc. Depending on the type of application, a single molecular sieve may not possess all of the properties desired for the application.
Composite materials containing two or more crystal structures, can be valuable materials because they allow for two distinct regions of activity or adsorptivity in a single unified structure. However, composite materials are quite rare in the literature.
U.S. Pat. No. 5,972,203 describes a catalyst comprising first alumino-phospho-molecular sieves and a binder comprising second alumino-phospho-molecular sieves. There is no evidence that the catalysts are intergrown, and the description is suggestive of two distinct phases with no coherency between them.
Zheng et al., “Synthesis of Self-Pillared Zeolite Nanosheets by Repetitve Branching,” Science, 336, 1684 (2012) describes zeolites having a MFI phase with MEL intergrowths. The MEL intergrowths introduce a different symmetry in the direction normal to the nanosheet, allowing growth (pillaring) in that direction. It relies on rotational intergrowth to produce the pillared nanosheets.
U.S. Pat. No. 8,809,217 ('217 patent) describes a catalyst for the selective reduction of NOx. The support for the catalyst is a molecular sieve having at least one intergrown phase having at least two different small-pore, three dimensional framework structures. The molecular sieves with intergrown phases are disordered as evidenced by the significant peak broadening in their x-ray diffraction patterns.
U.S. Pat. No. 8,163,259 ('259 patent) describes a molecular sieve comprising at least one intergrown phase of an AFX framework-type molecular sieve and a CHA framework-type molecular sieve. According to the '259 patent, intergrown molecular sieves are disordered planar intergrowths of molecular sieve frameworks. Structurally disordered structures show periodic ordering in zero, one, or two dimensions, rather than in three dimensions as with regular crystalline solids.
Disordered molecular sieves show weakened or broadened peaks in x-ray diffraction. For example, the peaks for the AFX/CHA material in the '259 patent are very weak and broad. This is also true of the '217 patent.
Even rarer are those materials termed coherently grown composites.
U.S. Pat. No. 8,846,998 describes a family of coherently grown composites of TUN and IMF zeolites. The structure was confirmed by x-ray diffraction, scanning electron microscopy, and transmission electron microscopy. In coherently grown composite structures, both structures are present in a major portion of the crystals in a given sample. This coherently grown composite structure is possible when the two zeotypic structures have nearly identical spacial arrangements of atoms along at least a planar projection of their crystal structure and possess similar pore topologies.
However, there are no known coherently grown composites of aluminophosphate and silicoaluminophosphate molecular sieves.